Partial discharges generally indicate all discharges that do not occur between electrodes. Such partial discharges cause electrical corrosion of an insulating material. For example, the partial discharges include a corona discharge that occurs near a pointed part of an electrode in a gas, a creeping discharge that occurs along a surface of a solid insulting material, and a void discharge that occurs in a void of an interior part or a surface of a solid insulating material.
The partial discharges may occur along an outer surface of an insulating material of an enameled wire. All the corona discharge, the creeping discharge, and the void discharge may occur depending on shapes of local parts of the insulating material of the enameled wire. The discharges are collectively called partial discharges or corona discharges in the art of the present disclosure. In most cases, the partial discharges occur due to an inverter surge applied to a motor controlled by a high-speed inverter including a pulse width modulation (PWM) method at an induction portion coil or an input terminal of the motor. The partial discharges destroy a polymer insulating layer of an enameled wire, which is a component of a coil. Further, the partial discharges cause shorts between neighboring wires and thereby disenabling the motor.
If the enameled wire has a polymer nanocomposite film on its skin, it is possible to accomplish high resistance against the partial discharges and to maintain mechanical and electrical properties such as electric insulation equal to those of a conventional enameled wire having only an organic polymer insulating film. In order to prepare the polymer nanocomposite, inorganic nanofillers need to be dispersed in a polymer matrix with high dispersibility. Since the partial discharges may be absorbed or reflected on surfaces of the inorganic nanofillers, the resistance against the partial discharges can be improved by enlarging the whole surface area of the inorganic nanofillers. If the inorganic nanofillers are condensed to form a cluster on the film of the enameled wire, electric fields converse on the portions where no inorganic nanofillers are distributed, or voids, so that the partial discharges cannot be prevented. Thus, the high dispersibility of the inorganic nanofillers should be maintained during all the processes for preparing the enameled wire.
In order to prepare a varnish in which the inorganic nanofillers are dispersed, for use in coating the enameled wire, three major methods have been conventionally used.
The first of the methods is a mechanically dispersing method, which directly disperses the inorganic nanofillers in the manner that the inorganic nanofillers are put in a polymer solution, and a strong shear force is applied thereto thereby mechanically homogenizing the condensation of the inorganic nanofillers. Since this method does not require chemical processing, preparing processes thereof are simple and inexpensive. However, if this method is used to disperse the inorganic nanofillers, it has been reported that there is a high possibility of occurrence of recondensation among the inorganic nanofillers, thus the dispersion stability is low. As a similar method, a colloidal solution is prepared in advance by monodispersing inorganic nanofillers that have not been subjected to surface-treating, in a certain organic solvent, and the colloidal solution is blended with a varnish. For example, Korean Patent Nos. 10-0756903 and 10-0656867 describes this method. However, this method has a limit in increasing a content of the inorganic nanofillers compared to polymer components. Accordingly, if a varnish in which the inorganic nanofillers are dispersed is prepared by the method and used for coating the enameled wire, satisfactory partial discharge resistance cannot be assigned to the enameled wire.
The second of the methods is a sol-gel method, which blends metal alkoxides such as tetraethoxysilicate with a varnish, and then, grows the inorganic nanofillers at a low temperature, or an in-situ polymerization method, which grows the inorganic nanofillers during processes for high temperature coating or heat processing. This method is advantageous in dispersing inorganic nanofillers in a relatively uniformed size. However, the electrical property of the enameled wire may be deteriorated after the enameled wire is coated due to unreacted metal alkoxides residual after reaction. Further, since reaction time is long, a speed of preparing processes is reduced, and thereby, increasing production costs.
The third of the methods relates to dispersing the inorganic nanofillers by performing surface-treating to the inorganic nanofillers with a silane coupling agent so as to enable the inorganic nanofillers to have affinity to a polymer solution. However, if this method is used, it is difficult to obtain satisfactory surface coverage of the silane coupling agent to the whole surfaces of the inorganic nanofillers. If complicated processes such as preparing and coating a varnish are performed by the surface-treating method that has been known to the present, the dispersibility of the inorganic nanofillers could not have been maintained.